This invention relates generally to optical systems, and more particularly to a method and system for steering a collimated light beam with a rotatable mirror.
Optical systems are used in a wide variety of contexts. One example is in communications systems where light is modulated to carry information. Optical carriers are useful due to the high bandwidth associated with the signals. As a result, telecommunications systems, at least for long haul applications, have become predominantly optical.
A relatively new technology that has been proposed for data communications is the optical wireless network. According to this approach, data is transmitted by way of modulation of a light beam, in much the same manner as in the case of fiber optic telephone communications. A photoreceiver receives the modulated light, and demodulates the signal to retrieve the data. As opposed to fiber optic-based optical communications, however, this approach does not use a physical wire for transmission of the light signal. In the case of directed optical communications, a line-of-sight relationship between the transmitter and the receiver permits a modulated light beam, such as that produced by a laser, to travel without the use of an optical fiber as a waveguide. Optical wireless communications is inherently secure because in order to snoop on the transmission, the transmission would need to be broken. A broken transmission link is readily detected.
It is contemplated that the optical wireless network according to this approach will provide numerous important advantages. First, high frequency light can provide high bandwidth, for example ranging from on the order of 100 mega bits-per-second (Mbps) to several giga-bits-per-second (Gbps), when using conventional technology. Additionally, this high bandwidth need not be shared among users, when carried out over line-of-sight optical communications between transmitters and receivers. Without other users on the link, of course, the bandwidth is not limited by interference from other users, as in the case of wireless telephony. Modulation can also be quite simple, as compared with multiple-user communications that require time or code multiplexing to permit multiple simultaneous communications. Bi-directional communication can also be readily carried out according to this technology. Finally, optical frequencies are currently not regulated, and as such no licensing is required for the deployment of such networks.
These attributes of optical wireless networks make this technology attractive both for local networks within a building, and also for external networks between buildings. Indeed, it is contemplated that optical wireless communications may be useful in data communication within a room, such as for communicating video signals from a computer to a display device, such as a video projector.
The present invention provides a configuration of components that can be used in an optical wireless network, as well as in a wide variety of other contexts. In a first aspect, an optical system includes a light source, such as a laser diode. A rotatable mirror is positioned to receive a light beam from the light source. A collimating lens is positioned to receive a reflected light beam from the rotatable mirror. The collimating lens preferably has a focal length that is about equal to the sum of the distance between the light source and the mirror and the mirror and the lens.
In another aspect, the present invention provides an optical system that includes a light collection device, such as a photodetector (e.g., a photodiode). A rotatable mirror is positioned to direct a light beam toward the light collection device. A lens is positioned to transmit a received light beam toward the rotatable mirror. The lens has a focal length that is about equal to the sum of the distance between the light collection device and the mirror and the mirror and the lens.
In these two embodiments, the area of the mirror does not limit the area of light that can be transmitted through the lens. The ability to direct light from a large area is beneficial in optical wireless communications. For the receiver, the ability to direct light from a large area means a large collection area can be focused to a small photodiode without restricting the field of view. For the transmitter, the ability to direct light from a larger area means a higher eye-safe output power and better collimated beam can be used.
In the preferred embodiment, a rotatable mirror is placed between a laser diode (or photodiode) and a lens such that the optical path distance from the lens center to the mirror to the diode is near the focal length. In this manner, the cross-sectional area of light entering or exiting the system through the lens is increased relative to the mirror area.